Switching and display devices which contain liquid-crystal mixtures have been used widely for years. Liquid-crystal displays (LCDs) are devices which, as a consequence of electrical switching, modify their optical transmission properties in such a way that incident--and possibly re-reflected--light is modulated in intensity. Examples are the known watch and calculator displays or liquid-crystal displays in the OA (office automation) or TV sectors.
These LCDs--also known as LC cells--are constructed in such a way that a liquid-crystal layer is enclosed on both sides by layers which are usually, in the sequence starting from the LC layer, at least one alignment layer, electrodes and a limiting plate (substrate). In addition, they contain one polarizer if they are operated in guest-host or reflective mode, or two polarizers if the transmissive birefringence mode is used. The switching and display elements may contain further auxiliary layers, such as diffusion barrier or insulation layers.
The interaction of the liquid-crystal molecules with the wall surfaces offers an important method of controlling the basic alignment of the LC molecules throughout the cell. In a similar way as a molecule is aligned with its neighbor within a liquid-crystal film by interactions, vessel walls can have an aligning effect on the liquid-crystalline phases. The nature and pretreatment of the cell surfaces causes the molecules of a liquid-crystalline phase to be aligned with the long axes parallel (homogeneous), perpendicular (homeotropic) or tilted to the limiting surfaces.
The special limiting surfaces which cause the alignment of the LC molecules are known as alignment layers.
Such alignment layers are described, for example, by J. Cognard (MCCC, Supplement 1, 1982) for nematic liquid crystals and by J. S. Patel et al. (Ferroelectrics 59 (1984) 137) for smectic liquid crystals. Several problems occur in the alignment of ferroelectric liquid crystals compared with nematic ones. Firstly, the aim is to obtain a uniform layer structure, and secondly the two switching states of the molecules must be well stabilized, since they are different from the alignment direction.
Two important parameters of LC displays which are highly dependent on the alignment layers used are
a) the maximum brightness (transmission in the bright state), PA1 b) the maximum contrast (ratio between the maximum transmission in the bright state and the maximum transmission in the dark state); PA1 a) The maximum transmission in the bright state T.sub.bright is described to a good approximation for FLC displays, as is known, by equation (1): EQU T.sub.bright =sin.sup.2 (.pi..DELTA.and/.lambda.).multidot.sin.sup.2 (4.theta..sub.eff) (1) PA1 b) The contrast is the ratio between transmissions in the bright and dark switching states. At present, maximum contrast values of from 5 to 10 are specified for FLC displays. The reason for these values, which are too low for many applications, for example TV, lies both in an inadequate bright transmission and in an excessively high residual transmission in the dark switching state. The residual transmission is easily detectable between crossed polarizers from a liquid-crystal structure which appears bluish. PA1 a) at least one oligomer/polymer of a 6-membered cyclic sugar unit, some or all of whose free OH groups may be derivatized and which carries equatorial substituents in the 2-, 3- and 6-positions on the ring, and PA1 b) at least one compound from the group consisting of macrocyclic compounds, cryptands, coronands, podands, mercapto compounds and ionophores.
both are described in greater detail for the case of ferroelectric liquid crystals (FLCs):
where .DELTA.n=difference between the refractive indices (uniaxial approximation), d=thickness of the FLC layer, .lambda.=vacuum wavelength and .theta..sub.eff =effective tilt angle. In the ideal case, T.sub.bright =1 (or 100 %). Whereas the first of the two terms in equation (1) can be optimized relatively easily by matching .DELTA.n and d to the wavelength of visible light, the material-side optimization of sin.sup.2 (4.theta..sub.eff) causes problems since .theta..sub.eff is generally very much smaller than 22.5 .degree. ( the optimum value ).
Numerous inorganic and organic materials have already been disclosed for use in alignment layers. Very good results, particularly with respect to bright transmission and contrast, are achieved by means of silicon monoxide vapor-deposited obliquely. However, this requires a very expensive vacuum process, which is not very suitable for industrial mass production.
Significantly less expensive are alignment layers comprising organic polymers, such as polyimides, polyamides and polyvinyl alcohol, having rubbed surfaces; however, the bright transmission and contrast of FLC displays containing such alignment layers are still clearly inadequate, so there is a considerable demand for further materials for alignment layers.
The use of cellulose and starch or derivatives thereof as a material for alignment layers has already been disclosed (for example JP-A-63/070 227, JP-A-59/149 321, JP-A-03/100 520 and JP-A-02/262 622).
Furthermore, EP-A-0 453 966 discloses that alignment layers comprising organic polymers can be modified by means of an amphiphilizating, in particular macrocyclic compounds in order to improve the properties of alignment layers.